Find the odd one out: Square, Triangle, Rectangle, Sphere, Pentagon

Correct — C.
Rule: flat polygons against a solid.

Square, triangle, rectangle and pentagon are two-dimensional figures bounded by straight sides: 4, 3, 4 and 5.

A sphere is a three-dimensional solid with one curved surface and no sides → option (c).

Why the others are wrong

• (a)
  Square — A square is a flat four-sided polygon, the same kind of figure as the rectangle, triangle and pentagon.
• (b)
  Triangle — A triangle is a flat three-sided polygon. It has the fewest sides, but side count is not what sets one shape apart here.
• (d)
  Pentagon — A pentagon is a flat five-sided polygon. Having the most sides does not make it odd: it is a plane figure like the square and triangle.

Odd-one-out word lists are grouped by the property most of them share. Here four names are polygons: closed, flat figures with straight sides.

The fifth, the sphere, is a solid. It has length, breadth and height, and its surface is curved.

Side count does not help: it would single out the triangle (fewest) or the pentagon (most) with equal right, while dimension singles out the sphere alone.

Key facts

• A polygon is a closed plane figure with straight sides.
• Side counts: triangle 3, square 4, rectangle 4, pentagon 5.
• A sphere is a three-dimensional solid with a single curved surface.

Common traps

• Grouping by side count and picking the pentagon or the triangle
• Treating a sphere as a circle: a circle is flat, a sphere is solid

No directly related past PYQ was found.